Liquid crystal displays have the advantages of portability, low power consumption, and low radiation. Therefore, liquid crystal displays have been widely used in common daily life. Typically, a liquid crystal display includes a backlight module having a cold cathode fluorescent lamp as a light source. However, the cold cathode fluorescent lamp has high power consumption compared with other light sources such as light emitting diodes (LEDs). Therefore manufacturers are where practicable seeking to replace cold cathode fluorescent lamps with light emitting diodes as the light source of liquid crystal displays.
FIG. 7 is an exploded, isometric view of certain components of a conventional backlight module. The backlight module 10 typically includes a flexible printed circuit board 11, a plurality of light emitting diodes 13 arranged on a top surface (not labeled) of the flexible printed circuit board 11, and a light guide plate 15. The light guide plate 15 includes a light incident surface 151, and a light emission surface 152 perpendicularly connected with the light incident surface 151. The light emitting diodes 13 are located adjacent to the light incident surface 151 of the light guide plate 15. The flexible printed circuit board 11 includes a plurality of solder pads (not shown). The light emitting diodes 13 are soldered on the solder pads and thereby electrically connected to the flexible printed circuit board 11.
Referring also to FIG. 8, this is an enlarged, inverted view of one of the light emitting diodes 13. The light emitting diode 13 includes a light emission portion 137 and two electrodes 133. The light emission portion 137 is located at one side of the light emitting diode 13 that is adjacent to the light incident surface 151 of the light guide plate 15. The electrodes 133 are located at two opposite lateral sides of the light emission portion 137. Light beams emitted from the light emission portion 137 of the light emitting diode 13 enter the light guide plate 15 via the light incident surface 151.
Referring also to FIG. 9, this is an enlarged, front plan view of one of the light emitting diodes 13 on the flexible printed circuit board 11. When the light emitting diode 13 is soldered to the flexible printed circuit board 11, solder material is melted and covers solder pad areas A of the flexible printed circuit board 11. In particular, some of the solder material interposes between solder pads (not labeled) of the flexible printed circuit board 11 and the electrodes 133. That is, a gap D between each of the solder pads and the corresponding electrode 133 is generated. As a result, part of the light emission portion 137 of the light emitting diode 13 may be located above the light emission surface 152 of the light guide plate 15. Therefore some light beams emitted from the light emitting diode 13 may not enter the light incident surface 151 of the light guide plate 15. This reduces the emitting luminance of the backlight module 10.
FIG. 10 is a chart showing a relationship between a relative emitting luminance of the backlight module 10 and a relative distance of each of the electrodes 133 of the light emitting diode 13 from the flexible printed circuit board 11. Horizontal coordinate values of the graph represent the relative distance of each electrode 133 of the light emitting diode 13 from the flexible printed circuit board 11. If the electrodes 133 of the light emitting diode 13 contact the flexible printed circuit board 11 directly, the relative distance is defined as 0. If the electrodes 133 of the light emitting diode 13 are substantially above the light incident surface 151 of the light guide plate 15, the relative distance is defined as 1. Vertical coordinate values of the graph represent the relative emitting luminance of the backlight module 10. If the backlight module 10 has a smallest possible luminance, the relative emitting luminance is defined as 0. If the backlight module 10 has a largest possible luminance, the relative emitting luminance is defined as 1. It can be seen that the greater the value of the relative distance, the lower the value of the relative emitting luminance. That is, the greater the distance of the electrodes 133 of the light emitting diode 13 from the flexible printed circuit board 11, the lower the emitting luminance of the backlight module 10.
Furthermore, when part of the light emission portion 137 of the light emitting diode 13 is above the light emission surface 152 of the light guide plate 15, this is liable to increase the thickness (height) of the backlight module 10.
What is needed, therefore, is a light emitting diode and a backlight module employing the light emitting diode which can overcome the above-described deficiencies.